Multiple-electrode, directional, acoustic source

ABSTRACT

Concentric electrode pairs of opposite polarity improve the efficiency of a spark-gap acoustic source for marine seismic profiling. One electrode of a pair is tubular; the other is rodlike and positioned axially within the tubular electrode. Among the benefits resulting from the concentric electrode configuration are constant output, directional control, high frequency, efficiency, and a high repetition rate.

ilnited States Poston, Jr.

[ MULTIPLE-ELECTRODE,

DIRECTIONAL, ACOUSTIC SOURCE Adolph M. Poston, Jr., Petaluma, Calif.

The United States of America as represented by the Secretary of theInterior Filed: Apr. 30, 1970 Appl. No.: 33,453

Inventor:

[73] Assignee:

1.1.5. C1 ..340/l2 SD, 181/.5 EM, 181/.5 XC Int. C1. ..G0lv l/00 Fieldof Search ..340/12 SD, 7 R;

181/.5 EM, .5 XC

[56] References Cited UNITED STATES PATENTS 7/1924 Hammond, Jr...'.....340/12SD 11/1953 Butler ..340/12SD 1451 Apr.17,1973

3,245,032 4/1966 Knott et a1 ..340/12 SD 1,758,993 5/1930 Wolff i.....340/12 SD 3,283,294 11/1966 Schrom ....340/12 SD 3,537,542 11/1970Droyan et a1 ....340/12 SD 3,286,226 11/1966 Kearsley et a1. .181/.5 EM3,416,128 12/1968 A11en ....340/12 SD 3,588,580 6/1971 Vining ..340/12SD Primary ExaminerBenjamin A. Borchelt Assistant E.xaminer1-1aro1dTudor Att0mey-Emest S. Cohen and Gersten Sadowsky [5 7] ABSTRACTConcentric electrode pairs of opposite polarity improve the efficiencyof a spark-gap acoustic source for marine seismic profiling. Oneelectrode of a pair is tubular; the other is rod-like and positionedaxially within the tubular electrode. Among the benefits resulting fromthe concentric electrode configuration are constant output, directionalcontrol, high frequency, efficiency, and a high repetition rate.

1 Claim, 3 Drawing Figures MULTIPLE-ELECTRODE, DIRECTIONAL, ACOUSTICSOURCE BACKGROUND OF THE INVENTION Seismic profiling is a technique fordetermining the nature and thickness of geologic structures. In atypical marine seismic profiling operation, a vessel tows an acousticsource and a receiving hydrophone through a body of water. Sound wavesgenerated by the acoustic source are transmitted through the water tothe sediment layers below. Depending upon sediment thickness andstructure, the incident waves reflect with varying intensity and spacingto the receiving hydrophone. In response to these reflected waves,electrical impulses are generated by the hydrophone and sequentiallyrecorded in a correlated pattern representing the submerged geologicstructure.

Among the many perameters affecting quality in seismic profiling is thenature and precision of the sound waves generated by the acousticsource. If the incident waves fluctuate in frequency and intensity, thereflected waves are correspondingly deformed. When the signalrepresenting the reflected wave is recorded, these wave deformitiesambiguously distort the seismic information. Prevention of this unwanteddistortion requires an acoustic source capable of producing sound wavesof precisely controlled frequency, duration, and intensity.

DESCRIPTION OF THE PRIOR ART Numerous acoustic sources are available forcontinuous seismic profiling. Among them are piezoelectric crystals, gasexploders, air guns, and spark-gap sources. Each acoustic source hascharacteristic advantages for specific problems; and each hasdisadvantages, even while functioning at optimum efficiency. Because ofmany inherent advantages, the spark-gap source, in particular, is anappropriate subject for improvement toward optimum capability.

Prior spark-gap sources are primarily intended for deep to very deepacoustic penetration of the seafloor. For this purpose they produce highenergy and low frequency components in their acoustic wave fronts.

The high energy and low frequencies, however, are completely unsuitablefor studying shallow water and thin sediment sections. In shallow waterand over small geologic structures, high energy spark-gap sourcesproduce spurious multiple reflections which degrade the quality ofrecorded data. Furthermore, the rapid high frequency acoustic dischargerequired for high resolution of shallow structures and thin beddedsediments is not possible with high energy sources.

Adding to their unsuitability for some seismic profiling applications,prior spark-gap sources are generally inefficient and inconvenient touse. lnefflciency results, in part, from their characteristicomnidirectional acoustic wave generation. Only a fraction of the totalwave energy generated by prior spark-gap sources is usefully directedfor reflection to the receiving hydrophone. The remainder is dissipatedin all directions in the surrounding water. Since much of the acousticenergy is wasted, cumbersome high energy electric power supplies arerequired to produce sufficient energy for optimum results,inconveniently limiting operation of these inefficient acoustic sourcesto use with large vessels.

0 lapse. Sound waves produced in this manner lack the predictableprecision necessary for accurate, high resolution, seismic profiling.

SUMMARY OF THE INVENTION This invention is an acoustic source for marineseismic profiling. When submerged in a conducting fluid, the acousticsource generates an acoustic pressure pulse each time sufficientelectrical potential appears between oppositely polarized pairs ofelectrodes. One electrode of each pair is a cylindrical tube, open atone end. The other electrode is a rod-like, wire filament, axiallypositioned within the tube, and surrounded by electrical insulation,except for a small exposed portion near the open end of the tube. In thepreferred embodiment, a linearly aligned series of electrode pairs ismounted on a tubular support. During seismic profiling, the acousticsource is suspended horizontally in sea water by a float and towedbehind a vessel, with the open end of each tubular electrode pointingdownward.

The electrode configuration used in this invention produces a superiorresult in comparison to prior spark-gap acoustic sources. Alignment ofthe concentric electrode pairs is preserved even though erosion of theinner electrode occurs. As a result, a constant intensity spark ismaintained throughout operation of the acoustic source. The restrictedvolume within the tubular electrode limits the size of bubbles formed atthe spark-gap, controlling the intensity of the resulting acoustic wave.Highly directional wave propagation results from the downwardorientation of the electrode pairs.- A resonant organ-pipe effect duringpulse generation in the electrode tubes results in higher frequencypulses than possible with conventional spark-gap electrodes. Because oftheir directional uniformity, pulses generated by the adjacent electrodepairs, contribute a uniform additive effect to the resultant acousticwave. The acoustic power of the wave is variable by simply changing thenumber of electrodes employed. The frequency of the wave is variable bychanging the dimensions of the tubular electrodes.

Through a superior electrode configuration, this invention enables ahigh repetition rate acoustic discharge with a constant output intensitysuitable for seismic profiling in shallow marine environments. As itoperates efficiently with minimal power requirements, the invention issuited for operation behind small vessels with lightweight powersupplies.

Therefore, one object of this invention is an acoustic source with aconcentric electrode configuration.

Another object of this invention is an acoustic source with an electrodeconfiguration resulting in constant output regardless of normalelectrode erosion.

Another object of this invention is an acoustic source with tubularelectrodes that control bubble formation and wave intensity.

Another object of this invention is an acoustic source with a tubularelectrode configuration for generating a directionally controlledacoustic wave.

Another object of this invention is an acoustic source for generatinghigh frequency waves through a resonant organ-pipe effect.

Another object of this invention is an acoustic source having an alignedseries of electrodes for generating an additive acoustic wave.

Another object of this invention is an acoustic source having a variablehigh frequency wave component.

Still another object of this invention is an efficient acoustic sourcehaving a high repetition rate and constant intensity output whenoperated with minimum power input.

These and other objects of the invention will be apparent in thefollowing specification and drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a lateral view of a typicalseismic profiling operation.

FIG. 2 is a partially sectioned lateral view of a multiple-electrode,directional, acoustic source.

FIG. 3 is a partially sectioned lateral view of a rodlike electrode,shown generally in FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT A typical seismic profilingoperation is shown in FIG. 1 where a vessel tows an acoustic source 12and a receiving hydrophone 14 through a body of water 16. Electricalcables 18 and 20 connect the source and hydrophone to electroniccomponents 22 within the vessel. The hydrophone 14 floats freely in asubmerged position behind the vessel, tethered by connecting cable 20,while the acoustic source 12 rides just below the water surface on afloat 24. Stabilizing fins (not shown) and a torpedo-like shape maintainthe float 24 and suspended acoustic source 12 in a horizontalorientation, substantially paralleling the bottom surface 26 below. Aflexible towing cable 28 joins the float 24 and vessel 10, orienting theacoustic source 12 parallel to the direction of travel as the vesseladvances through the water.

For measuring the layers of a geologic structure 30, sound waves 32 aretransmitted from acoustic source 12 toward bottom surface 26. The soundwaves reflect from the bottom and subsurface layers to receivinghydrophone 14, where their intensity and time of arrival are detected.Representative electrical impulses from the hydrophone travel alongconnecting cable 20 to electronic recording equipment in the vessel,making a permanent record of the seismic profile. Depending upon thenature and thickness of a reflecting layer the reflected wave varies inintensity and time of arrival. As the vessel proceeds, correlation ofthe recorded information from discrete reflected waves produces anaccurate seismic profile, showing intensity variations by shadinggradations, and arrival time by spaced separation of the seismicinformation across the recording sheet.

In the seismic profiling operation shown in FIG. I, only those soundwaves 32 passing through a narrow incident window are eventuallyreflected to the receiving hydrophone. Sound waves incident in otherdirections (not shown) are reflected out of receiving range. Bydirecting a large percentage of the sound waves from the acoustic sourcethrough the incident window, inefficient dissipation of acoustic energyin areas away from the hydrophone is avoided. For this purpose, adirectional acoustic source is desirable.

In FIG. 2 a directional acoustic source 12 is shown in detail.Basically, the source 12 has a long tubular housing 40, on the side ofwhich are mounted a linearly aligned series of downward projectingcylindrical tubes 4252. Both the housing and tubes are electricallyconductive, each individual tube acting as one electrode of a spark-gappair. Conccntrically positioned within each tube is a second, rod-likecentral electrode 54, as shown by the sectioned representation of tube46. When the acoustic source 12 is immersed in a conducting fluid, suchas salt water, and sufficient potential difference applied between theinner and outer electrodes, an electrical discharge, or spark, occurs inthe gap between the electrodes. Fluid entering the gap through the openend of the tube, and through relief openings 56, is heated by thedischarge, forming a rapidly expanding gaseous bubble. The generationand ultimate collapse of this bubble sends a sharp pressure pulsetraveling through the conducting fluid. By applying a continuouspulsating potential difference between the electrode pair, a continuousacoustic wave is generated.

At opposite ends of the acoustic source 12, domed end-caps 58-60 sealthe housing 40 to prevent fluid seepage from interfering with electricaloperation of the electrode pairs. Electrical cable 18 enters theinterior of housing 40 through a sealed opening 62 in end-cap 58. Oneconductor 64 of cable 18 is electrically coupled to housing 40 by aconnecting clip 66. The other conductor 68 is electrically coupled to abus bar 70 by a similar connecting clip 72. Since the housing 40 andelectrode tubes 42-52 are all conductive, any potential differencebetween conductors 64 and 66 is transferred to the electrode tubes andbus bar 70. The inner electrode of each pair is insulated from thehousing and surrounding tube, and coupled to the bus bar, making theinner and outer electrodes of opposite polarity corresponding to anypotential difference between the conductors.

The rod-like inner electrode 54 is shown in detail in FIG. 3. A similarrod-like electrode is axially aligned within each electrode tube, eventhough, for descriptive convenience, only one inner electrode is shownin FIG. 2. Because the inner electrode gradually erodes with continueduse, it is constructed in two sections, one 74 permanent and the other76 replaceable. On the permanent electrode section 74 an insulatingsupport 78 surrounds the central portion of a wire conductor 80. Theconductor protrudes from the support at one end 82 where it joins thebus bar 70, and at the other end 84 where it joins the replaceableelectrode section. For attaching the permanent electrode sections 74 tothe housing 40, a mounting plate 86 is fixed at the closed end of eachtubular electrode, as seen in FIG. 2. In the plate 88, a threadedaperture aligns with apertures through the housing and bus bar 70. Athreaded shank 90 secures the permanent electrode section 74 within theaperture 88, while a raised shoulder 92 seats within a counter-bore 94on the mounting plate, insuring accurate positioning and a fluid tightseal. For electrical continuity the conducting wire 8% is soldered tothe bus bar 70.

On the end of permanent electrode section 74 opposite the threadedshank, there is a smooth shank 96 encircled at one point by a narrowraised bead 98. The smooth shank and raised bead cooperate with aflexible elastic tube 100 to join the permanent and replaceableelectrode sections into a sealed structural unit.

In basic structure the replaceable electrode section 76 resembles thepermanent section, having a central wire conductor 102 surrounded by aninsulating sleeve 104. Near one end, sleeve 104 is encircled by a narrowraised bead 106, similar to bead 98. Within the same end of sleeve d isa broad end portion 108 of central conductor 102, with a deep axial bore1 10. The diameter of bore 110 corresponds to the diameter of conductingwire 80 to insure a tight conductive joint when the permanent andreplaceable electrode sections are assembled. To assemble the sections,one end of elastic tube 100 is slipped over raised bead 106, and the end84 of conducting wire 80 is inserted into bore 110, while elastic tube100 is slipped over raised bead 98. Axial alignment of the sections isinsured by tight mating contact of conductors 80 and 102, andlongitudinal alignment is insured by elastic tube 100. Fluid seepageinto the joint between the sections is prevented by the tight elasticgrip provided by the tube.

In operating position, the free end of a rod-like central electrode 54terminates in alignment with the free open end of each tubularelectrode, as seen for tubular electrode 46 in FIG. 2. The tip 112 ofconducting Wire 1102 projects slightly from insulating sleeve 104 in anaxial position radially equidistant in all directions from the wallsurface of the electrode tube. When sufficient potential difference isapplied between the electrodes submerged in a conducting fluid, auniform electric current radiates between tip 112 and the wall of theouter electrode. Continual application of a potential difference causesuniform longitudinal erosion of tip 122 and the adjacent portion ofinsulating sleeve 104. Because the erosion is longitudinal, the gapbetween tip 112 and the outer electrode wall remains constant, and theradiating electrical current effectively unchanged. When the conductingwire and insulating sleeve erode past a useable length, the innerelectrode is easily replaced.

In addition to maintaining a constant gap interval, the downwardprojecting electrode tubes 42-52 limit the size of bubbles formed at thespark-gaps, and direct the acoustic wave down toward the geologicstructure below. As explained above, uniform bubble generation iscritical to producing consistent, uniform acoustic pressure pulses;directional control of the acoustic wave is critical to efficientoperation of the acoustic source.

An acoustic wave of higher frequency than produced by prior spark gapsources is another advantage of the tubular electrode configuration,resulting from a resonating organ-pipe effect that occurs during bubblegeneration. In theory the frequency of a closed tube is given by theformulazf V/4L, in which:

f fundamental frequency in cycles per second.

L tube length in inches.

V= velocity in inches per second.

in sea water the velocity V is approximately 5.76(1O) inches per second.For a tubular electrode 3 inches long the fundamental frequency is:

f 5.76( l0)/(4)(3) =4,800 cycles per second.

This high frequency component is changed by simply varying thedimensions of the tube. As this relatively high frequency is achieved inthe acoustic source 12 by a mechanical effect, independent of theelectrical power applied, a tubular electrode configuration results in asubstantial increase in efficiency in comparison to prior spark gapsources.

For varying seismic profiling applications, the acoustic source 12 isalternately constructed with any number of electrode pairs. Increasingthe number of electrode pairs produces an additive effect, yieldingmaximum acoustical energy for a given electrical energy input. Theindividual acoustic waves, produced simultaneously by each electrodepair, combine into a single front as they travel away from the source.Because each individual wave is precisely directed in line with theother waves, the additive effect is complete, with minimal loss ofacoustic energy in the combined wave front.

Actual seismic profiling, as described with reference to FIG. ll, isperformed by submerging the acoustic source in sea water 16 behindtowing vessel 10. An electrical cable 18 connects the acoustic source toelectronic components 22 within the vessel. These components include anelectrical energy source similar to those used in the prior art. Onecommonly used energy source is a capacitor bank charged by a highvoltage d.c. generator. A trigger circuit connects the capacitor bank tothe acoustic source at timed intervals, discharging the capacitorsthrough the gaps between the electrode pairs, and generating an acousticpressure pulse.

Seismic profiling in shallow water requires a high repetition rateacoustic discharge with a constant output intensity. To maintain aconstant output intensity the capacitor bank must charge fully beforeeach discharge. As system efficiency decreases, the size of thecapacitor bank increases, and more current is required for fullcharging. A powerful generator is necessary to supply the requiredcurrent in the short time essential to a high repetition rate. Becauseof space and weight restrictions on vessels operating in shallow water,a high repetition rate is possible only with an efficient acousticsource yielding a maximum acoustic output from a less powerful,lightweight electrical source. The directional, multi-electrode acousticsource shown in FIGS. l-3 yields a high repetition rate acousticdischarge with a constant output when powered by a relatively smallelectrical power supply. For this reason, it enables seismic profilingfrom small vessels in previously inaccessible areas.

Although, for convenience, this invention is described by reference to asingle, specific, preferred embodiment, numerous modifications withinthe scope of the invention are expected. For example, a circularelectrode array might be substituted for the linear array. Square orhexagonal tubular electrodes might be substituted for the cylindricalelectrodes, and small concentric tubes for the wire electrodes shown.Sequential electrode discharge might be substituted for synchronousdischarge. Sequential discharge of a vertically oriented acoustic sourcemight be used to generate additive waves in a narrow, concentratedfrontal area. These and other modifications will be obvious to theskilled worker in the art. For this reason, the invention is limitedonly by the scope of the following claims.

lclaim:

1. An acoustic source comprising:

a conducting tubular electrode, open at one end,

closed at the other end, and having at least one relief opening betweenthe ends to enable ambient fluid to flow into or out of the electrode,

a conducting rod-like electrode of smaller diameter than the tubularelectrode, positioned concentrically within, and extending substantiallythe entire length of the tubular electrode to a point beyond the reliefopening,

means for supporting the rod-like electrode near the closed end of thetubular electrode,

means for electrically insulating the tubular and rodlike electrodesfrom one another at the means for supporting, and v conducting means forapplying an electrical potential difference between the tubular androd-like electrodes.

1. An acoustic source comprising: a conducting tubular electrode, openat one end, closed at the other end, and having at least one reliefopening between the ends to enable ambient fluid to flow into or out ofthe electrode, a conducting rod-like electrode of smaller diameter thanthe tubular electrode, positioned concentrically within, and extendingsubstantially the entire length of the tubular electrode to a pointbeyond the relief opening, means for supporting the rod-like electrodenear the closed end of the tubular electrode, means for electricallyinsulating the tubular and rod-like electrodes from one another at themeans for supporting, and conducting means for applying an electricalpotential difference between the tubular and rod-like electrodes.